It should be used for a true earth connection, but the fact is that you will find it commonly used as common in many, many circuit diagrams. Always be careful, but remember that battery based circuits very rarely need a ground connection, so you should be safe if the same is used.
Minder has provided the correct symbol denotations. Also, remember that the digital/analogue ground may be marked with an A or D to represent ones relevant to Analogue or Digital respectively. These should be used if no connection to a ground rod or a large metal body or chassis is present.
It is incorrect, but it has become informal convention among many.
*Edit: A little off topic, but I thought I would mention that a chassis or earth connection can be important for a few reasons. A ground connection can prevent the build up of charges in circuits, the (usually) moist soil of earth dissipates any of these potentially destructive charges. A more novel use is the potential difference created between an aerial and a ground connection, with a rectifier and a capacitor, an LED can be made to briefly light if touched across the capacitor. An interesting demonstration of the power of electromagnetic radiation surrounding us constantly. Other uses include RF dissipation. Of course, one of the most noticeable uses is that of safety. The chassis of mains appliances can be earthed, therefore if the chassis becomes hot, due to a short, the dangerous live current has a less resistive path to ground. This has the obvious effect that the path to earth (usually <1Ω), diverts the current away from the person that comes into contact with the chassis.